One concept for a high-efficiency LED-based illumination system with improved color rendering is three-color mixing. In this concept, the mixture of the primary colors red-green-blue (RGB) is used to generate white. A blue LED can be used for partial conversion of two phosphors which emit red and green. The search for an efficient green phosphor for an RGB system is currently at the forefront of research, as is demonstrated, for example, by the proposal from U.S. Pat. No. 6,255,670. Alternatively, a UV-emitting LED which excites three phosphors respectively emitting in the red, green and blue is used, cf. WO 97/48138. Examples include line emitters, such as YOB:Ce, Tb (green) and YOS:Eu (red). This requires a relatively short-wave emission (UV region <370 nm) to enable high quantum efficiencies to be achieved. This requires the use of sapphire substrates for the UV-LED, which are very expensive. On the other hand, if a UV-LED based on the less expensive SiC substrates is used, one has to be satisfied with an emission in the range from 380 to 420 nm. The individual colors of the RGB system can in principle be generated by the primary radiation of LEDs or by luminescence conversion LEDs, as is illustrated by WO 01/41215.
To increase the overall light yield, a complicated system including a fourth LED which emits in the range from 575 to 605 nm has also been proposed in WO 00/19141. By its very nature, a system of this type is significantly more intricate, expensive and complicated than an RGB system.
DE-1 A 101 37 042 shows a slightly different concept, proposing a planar illumination system with special introduction of the blue component in order to avoid the usual absorption problems encountered with blue phosphors.
One interesting class of phosphors for illumination systems of this type are those of the oxynitridosilicate type, as are known per se under the shortened formula MSiON; cf. for example “On new rare-earth doped M-Si—Al—O—N materials”, J. van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4, Chapter 6. They are doped with Tb. Emission is achieved under excitation by 365 nm or 254 nm.
A new type of phosphor is known from the as yet unpublished EP patent application 02 021 117.8. It consists of Eu- or Eu,Mn-coactivated oxynitridosilicate of formula MSi2O2N2 (M=Ca, Sr, Ba).